docs: update examples for response accessor API change

Updated code examples to reflect the breaking change in probitas-client
where response body accessors changed from methods to properties.

Changed accessor calls:
- res.json() → res.json (HTTP client)
- res.data() → res.data (gRPC/GraphQL clients)

This aligns with probitas-client PR #65 which converted getter methods
to properties for cleaner API. The json property now returns 'unknown'
requiring explicit type assertions where needed.

Ref: probitas-test/probitas-client#65

Author: lambdalisue

docs/client.md

@@ -365,7 +365,7 @@ for await (
   )
 ) {
   expect(res).toBeOk();
-  console.log("Received:", res.data());
+  console.log("Received:", res.data);
 }
 ```
 

docs/configuration.md

@@ -273,7 +273,7 @@ scenario("Retry Example")
         },
       },
     });
-    return res.json();
+    return res.json;
   })
   .build();
 ```

docs/scenario.md

@@ -31,7 +31,7 @@ export default scenario("User API CRUD", {
     const { http } = ctx.resources;
     const res = await http.get("/users/test-user");
     expect(res).toHaveStatus(200);
-    return res.json();
+    return res.json;
   })
   .build();
 ```
@@ -553,7 +553,7 @@ scenario("Retry Example")
       const { http } = ctx.resources;
       const res = await http.get("/sometimes-fails");
       expect(res).toBeOk();
-      return res.json();
+      return res.json;
     },
     {
       retry: { maxAttempts: 3, backoff: "exponential" },